Gaseous heat exchange



GASEOUS HEAT EXCHANGE Las W sa/ g5 i gg INVENTOR E o u LEONARD P. Pool. 5 E zr E l lu El', E L BY ATTORNEY Nov. 24, 1953 l.. P. Pool. 2,660,038

GASEOUS HEAT EXCHANGE Filed March 3, 1950 3 Sheets-Sheet '.2

Nov. Z4, 1953 L. P. Pool.

GASEOUS HEAT EXCHANGE 3 Sheets-Sheet 3 Filed March 3, 1950 ATTORNEY atentecl Nov. 24, 195.3

GASEOUS HEAT EXCHANGE Leonard Parker Pool, Allentown, Pa., assignor to Air Products, Incorporated, a corporation of Michigan Application March 3, 1950, Serial No. 147,419

21 Claims. (Cl. (i2- 123) This invention relates to a method and apparatus for the separation of gas mixtures containing components of different boiling points, including the preliminary removal of one or more undesirable high boiling constituents from the mixture. More particularly, the present invention relates to a method and apparatus for the separation of air into an oxygen-rich fraction and a nitrogen-rich fraction including the elimination from the air of undesirable impurities, such as water vapor, carbon dioxide and other rhigh boiling components prior to fractionation.

The separation of air, or other low-boiling gas mixtures, into the relatively pure major components has been accomplished by processes which include compressing and precooling the mixture, liquefaction of a` portion of the mixture by heat interchange with a cold product of the separation, the expansion of another portion with external work, fractionation of the two portions in a fractionating column, and utilization of the cold products of the separation for the precooling of the incoming compressed mixture. In processes of this character, so-called cold exchangers have been employed to precool the gas mixture by countercurrent heat exchange with the cold products of the fractionation. One type of cold exchangers employed has been switching type interchangers which permit a simultaneous and eiiicient heat interchange between passageways in the interchangers containing countercurrently flowing streams of air and cold products of the fractionation. This form of interchanger comprises a plurality of parallel paths for the uid forming each passageway which are so unlike the continuous ilow interchanger, these `.forms permit the impurities deposited in cooling the air to be, at least in large part, removed by sublimation or evaporation when the flows are rswitched and the cold gas flows through the pasf'sages'in which the impurities have been deposited.- This removal is accomplished by periodi- Z cally alternating the flow of warm incoming feed and a cold product of the fractionator between at least two passageways of the interchanger. That is, during one half of the cycle compressed air is being cooled in its passage through` one passageway of the exchanger in countercurrent exchange with a stream of nitrogen in a second passageway and a stream of oxygen in a third passageway. During this period of ow, impurities are deposited on the heat-exchange surfaces of the one passageway. After an appropriate period, say several minutes, a mechanically operated valve mechanism switches and thus reverses the iiow of air and nitrogen in the first two passageways. The compressed air now flows through the passageway previously handling nitrogen, and in reverse direction to that of the nitrogen. Nitrogen now iiows where air has just passed, and in opposite direction to that of the air. The entering nitrogen contains none of the higher boiling impurities normally found in air, and is at approximately atmospheric pressure. It, therefore, has considerable capacity for the impurities precipitated on the surfaces of the passageway, and will, in fact, vaporize and carry them from the passageway. At the end of an equal period, the passageway will be clean of impurities and ready for the switchover toreceive incoming air. The process in the second paissageway is like the iirst, and through such repeated switchovers, the air is purified andthe exchanger kept reasonably free from an accumulation of impurities. The oxygen stream is not switched and thus is brought out dry and clean. The exchanger will maintain itself free of stoppages if the material balance on impurities shows no accumulation in any section of the exchanger.v In other words, the amount of impurities carried out of any section by the leaving streams must be equal to the amount of impurities carried into that section by the entering streams. This is accomplished only if the conditions influencing the complete re-evaporation of the impurities are maintainedthroughout the regions of the apparatus containing the deposits of these materials. These conditions relate specifically to the provision of a suicient volume of gas into which the deposits can be evaporated andrcmoved, and to the maintenance of a sufficiently high vapor pressure ofthe deposited impurities which latter is governed by the temperature of the cold product in the region of the deposited impurities. The relative vapor pressure characteristics of water vapor are such that no difiicultyis found in removing the de posited water at the pressure of the purging gas. However, in the case of carbon dioxide, the vapor pressure characteristics are such that it is diicult to obtain complete re-evaporation of the deposited carbon dioxide. Heretofore, the carbon dioxide has been substantially completely reevaporated V*by passing ta greater -quantity of cold product-.gases l,over-the deposited impurities :than the quantity of air from which they were deposited. One way in which this condition has been attained has been by introducing into the system an additional quantity of high pressure air which has been previously puried by chemical means. This results a'larger `quant-ity of product gases being availableforftheprecooling step than the quantity of air which lays down the impurities in the pre-coolingstep; yand similar systems for actually, or in effect, increasing the quantity of returning products over the' yamount of incoming air achieve the desired rel-vsul-ts 'by the temperature Vapproach principle, Cwhereinthe'temperature oftheair in .the `cold end lof the interchanger 4is `brought intocloser V-rirxfimity with the temperature -of 'the 4products with-which it is-in heat exchange relation. @Maintaining a small -diierence `in these temperatures yresults in-a small difference vbetween the zvapor pressures of the impurities at the time of their "precipitation andre-evaporation, and by maintaining the v-ratio of these vapor pressuresequal *to the-,ratio of the volume of air .tothevolume "of nitrogen, the nitrogen thus becomes saturated -With-the `impurities and is capable of removing @substantially all of the impur-ities from the rsysat the "same-rate Vat-'which they havev been depositedwithin the system.

rIlhe present invention utilizes a -new concept fori-accomplishing the complete re-evaporation of thedeposited impurities-namely, maintaining `v'the-hit-rogen `flowing through lthe zone 4w-herethe limpurities have been deposited at a temperatureapproximately equalr to or higher than the -temperature ofthe air at the time the impurities -Wer-jdeposited, as more particularly .set out here- It is-an'fimportant object of this invention to 'provide for -a' substantially complete re-evapora- -tion 4ofthe solidiiied higher boiling impurities depo'sitedin a switching cold interchanger.

Itfis'afiurther important object of this inven- 't-ionto'provideffor warming the purging gas prior `to#its-*coifltactwith the s olidined impuritiestofa temperature approximately Aequal to or `higher -than "the temperature at which the impurities `Awere deposited so that substantially all of the `solidified impurities are removed by the flow of "the purging gas.

-Itis fa still further important object of the #invention to provide Va method and apparatus in vwhich the impurities depositing vzone is mechanically shifted to a warmer zone of the interchanger with each reversal of the flow of the gases through the interchanger.

Fig. 1 is a diagrammatic View of one form of the invention,

Fig. 2 is a diagrammatic View of a second form of the invention, and

Fig. 3 is a diagrammatic view of a third form of the invention.

Referring rst to Fig. 1, air enters the system Y,at .L and is substantially'ireed from .dust in an air cleaner Il. This element maybe an electrostatic precipitator, a scrubber, or a simple air filter. The cleaned air passes at l2 to a compression `unit consisting of a steam turbine or other power source. I3, a first and a second stage turbociornpressor :I- li and l5, a water-cooled intercooler i6,andanaftercooler I1. The compressed air leaves A`the aftercooler via conduit I8 at a ,pressure the neighborhood of 7 atmospheres .through the switching valve `32, conduit 43,

75 inneren@ @passageway '.27 ,is @consisted t0. .c n

'section 520 andl atcoldsection .2 l Vsection and lthe cold section .of the .heat inter.- -changer comprise three passageways,.2 2, .23 and 24 theWar-m section, ldiagrammatically illusand `a temperature of about 300 Kelvin. The Asystem includes an automatically switching heat interchanger I9 which is divided into a Warm Both .the iwarm trated .as concentric :to one another, .and 26, 21

Vand .2s .of similar conguration inthe .cold `section thereof. Theoold section of `the .heat vinterchanger is Adesigned so as .to cool the incoming "air overa rangeiabout Vdouble .the carbon dioxide deposition range, so that'virtually all of .the

' carbon dioxide impurity is deposited at .thecold .end of this section. `-Switching valves29, 30, 3| and V32 are provided zto direct .the Vflow .of the Y iiuids. These valvesare rotated through .quarter .turns -in .synchronismandat suitable intervalsby Vsuitable means shown .only `in diagrammatic form. 'The form of valve indicated .is .merely illustrative of one type of valve means that Acould v be used.

With these valves in .the position shown inY Figure 1, conduit VI 8 is connected toconduit :33 so-that-the incoming air passes through .conduit i8, valve 129, .conduit '33 and .thence v#through Yiziassagewayll in the warm sectionfil -of the heat interchanger.

The.. air leaves .passageway 24' through `conduit .34, passing. through 'switching valve .313, andconduit to the lower end .of passagewayiZS in the coldsectionl of A`therheat intel-changer. The air leaves passage- -i1va-y2ijthrough.conduites, again passing through switching -.valve'3.0, and thence through conduit The oxygen productifrom oxygen product passes out of the system through vconduit '4L `itshould be noted that theoxygen product vas illustrated always `nowsV through the same passageways in the same direction through "both heat interchanger sections. lHowever, ,min

L1OW10 the oxygen in thecolder sectionl S0 @manages it ,be ,more desirable to reverse the that it' always ..ilQWs countercurrently to the 341' The nitrogen product from the fractionation operation flowing through conduit .42 is passed duit 45 leading to the switching valve 3| and thence to conduit 46 leading to the lower end of passageway 23 in the warm section of the heat interchanger. The upper end of passageway 23 is connected to conduit 41, switching valve 29 and outlet conduit 48.

When the valves are simultaneously rotated a quarter turn, thefunction of the described conduits are interchanged. The ow of the nitrogen gas will now be through conduit 42, valve 32, conduit 31, Valve 3l), and conduit 35, leading to the lower end of passageway 28in the cold section of the heat interchanger. It should be noted that this passageway 28 previously carried the air which flowed in the same direction through the passageway. Prior to rotation of the valve, the nitrogen vowed through the cold section entering at the top of passageway 21 and leaving at the bottom thereof. In this manner, the cold end `of the cold section of the heat interchanger is alternated between the top, as at the rst setting of the valves, and the bottom upon rotation of the Valves. The nitrogen leaves the passageway 28 through conduit 36, valve 30, and conduit 34 to the bottom of the passageway 24 through the warm section 20 of the heat interchanger. The passageway 24 previously carried the air entering at the top and leaving at the bottom. It will thus be seen that in the operation of this apparatus, the warm section of the heat interchanger will always have its warm end at the top where the air will enter rst through passageway 24 and then upon rotation of the valve 29 through passageway 23. The nitrogen will always enter the warm section 29 through the bottom, rst through passage 23 and then upon rotation of the valves through passageway 24. With valves 29, 3|), 3|, and 32 in the second position, incoming air will pass through valve 29 into conduit 41 and passageway 23 of warm section 20, thence through conduit 46, valve 3|, conduit 44 to passageway 21 of cold section 2|. From the coldsection of the heat interchanger the air passes to the column through conduit 45, valve 3|, conduit 43, valve 32 and conduit 38. From the above it will be apparent that in the cold section of the heat interchanger, the upper end thereof will alternately be the cold end and then the warm end due to the fact that the cold nitrogen gas will rst enter the cold section through the top of passageway 21 and upon rotation of the valves 30, 3| and 32 will enter through the bottom of passageway 28 with the air owing countercurrent thereto. Thus, the carbon dioxide will deposit first at the upper end of passageway 28, and upon rotation of the valves as described, the carbon dioxide deposition zone will be shifted to the lower end of passageway 21.

The nitrogen, after rotation of the valves will flow into the lower end of passageway 28 and will be warmed by heat interchange with the air owing downwardly through passageway 21, so that by the time the nitrogen reaches the previously deposited carbon dioxide in the upper part of passageway 28, the nitrogen will have been ywarmed to a temperature which may be equal to or higher than the temperature at which the carbon dioxide was deposited therein. At the same time, the air cooled by the nitrogen will deposit virtually all of its carbon dioxide content in the lower end, now the colder end, of passageway 21. Upon again rotating the valves, the operations are interchanged, with the carbon di- Qxidddepositgd inthe passageway-21 being evaporated'by the warmed nitrogen flowing .therethrough.

The fractionating column generally indicated at 50 may be any conventional column. The form shown consists of a high pressure section 5| and a low pressure section 52, separated by Ia partition plate and a reuxing nitrogen condenser 53. Each of the sections is provided with bubble plates 54. The air receives a preliminary fractionation in the ,high pressure section 5|, and crude oxygen of more or less 40% purity collects in the base of the section as a pool 55, while more or less pure nitrogen vapor is partially condensed in the nitrogen condenser 53. The crude oxygen passes through conduit `56 and an expansion valve 51, and thence through conduit 58 to an intermediate point in the low ypressure section. The nitrogen vapors rising in the high pressure section of the column are condensed in the nitrogen condenser 53 by heat interchange with the pool of product oxygen 59 which collects at the bottom of the low pressure section of the column. A portion of the condensed nitrogen flows downwardly through the high pressure section of the column to provide reux therefor. 'I'he balance of the condensed nitrogen collects` in a pool 60 just below the nitrogen condenser. The high pressure liquid nitrogen collecting in pool 60'is withdrawn through conduit 6|, flowing thence through expansion valve 62 and conduit 63 to the upper end of the low pressuresection. The intermediates thus introduced are refractionated in the low pressure section. Gaseous low pressure nitrogen is withdrawn from the top of the column through conduit 64 iiowing through a liqueer 65 where it passes in heat exchange with and liquees a portion of the incoming air. The gaseous nitrogen leaves the liqueer y55 through conduit 42, above referred to, leading to the main heat interchanger I9. Oxygen in a desired state of purity collects around the nitrogen condenser 53 in a pool 59 which surrounds the tubes of the nitrogen condenser. In condensing the high pressure nitrogen vapor within the tubes of the nitrogen condenser 53, the oxygen surrounding the tubes boils and its vapor passes upwardly through the column. A portion of the vapor is removed through conduit 39 leading to the heat interchangers.

To provide make-up refrigeration for the system to replace cold lost through heat leakage and imperfect heat exchange a portion of the cooled compressed air flowing in conduit 38 is diverted at point B6 through conduit 61 controlled by valve 68 leading to .a turbo-expander 69. In the turbo-expander 69, the pressure of the air is reduced while doing work and the temperature of the gas is thus lowered. The turbo-expander 69 is coupled with a turbocompressor or electric generator 10 for utilizing the work developed in any desired manner. The expanded air stream passes through conduit 1| to an intermediate point 12 in the low pressure section of the column. At point 13 in conduit 38, a portion of the incoming air is diverted through conduit 14, controlled by valve 15, and passed to liqueer 65 where it is liquefied by heat interchange with the eiuent nitrogen product flowing from the column through conduit 64. The stream of liquefied air then passes through conduit 1B to the point 11 where it merges again with the remainder of the stream of incoming air passing through conduit 3 8 controlledby valveV 1 8. The merged lz stream with the properVA liquid to -yapor ratioithen`l passes into the high pressure section, of` the columnfa's indicated at 191.

In the operation f of` this: form'of theiinvention; itl willI be seen` thatthel warm: compressed air: will enter the upperl end of f thewarm' section of the-heat interchangerandithen will alternately enter'the-top and-bottom of thecold sectionof the1h`eat interchanger. Thevcarbon dioxide present inthe `'incoming air is depositedat-the cold endiof the `cold section'of the heat-interchanger which is, asdescribed, rst at-the-flootto1n` and then at the top. thereof; The cold nitrogen stream alternately enters the bottom\ and top `oi' the coldfsection ofthe heat interchangerthrough the passageway iniwhich the-carbon-dioxide has been:deposited.` Thisstream isv warmedibefore itlreaches` the carbon' dioxide deposit, Iso that byftlfpe time it reaches-the carbon dioxide deposit it isiat a4A temperature -approximatelyequal to origreater than'the temperature vat which the carbon f dioxide fwasf deposited and vhence, hasy no diculty insweeping the carbon dioxide out. Additionalfcoldis added to the system bypassing a` portion ofY the coldcompressed air from the interchanger through aA turbo-expander wherein `the temperature 'off the stream is vloweredq-and the stream r is then passed yto. the fractionating-.column Thecold section of :the heat Vinterchanger preferably coolstheair through aY-temperature range atfleast ftwic'e therange in which the carbon dioxide' is deposited. The Zfollowingis' af specific example-ofthe temperatures of uthestreams flowing in the: cold section of thef heat interchanger; Assuming-.the air iscoinpress'ed to approximately 7 VYatmospheres and contains 'in the' neighborhood of,:325fparts.per million of carborrfdioxide, the' carbon. dioxide i contentv of 1 the air will begin Lto' deposit out of the iair' at'a'ftemp'erature ofabout 142 K'fand will continueto deposit in measurable exten-t until cooled' to about 117? K; The:r air stream.A at the latter-1 temperature 'I willl lfravey a carbon-dioxide content'pf aboutparts-iper lmii-- lion,` which will riotbe deleterious Vto thewoperationf-of'- the system! With a mass volume* of returning product equal to 100% foithelincoming air used -forscavengingg the returning products' at approximatelyf 1 1/VV atmospheres *pressurewil-l remove'all of thedepositedoarbondioxide so long as Vtheternperature of Ythe products is aboveabout 132.6` K'.' when" the products first"A Contact the deposited carbon dioxide; This is dueto the fact that the' maximum allowable temperature dife' ference between the' air and nitrogen has been found to be 9.4 'K under 'such conditions. With 80%of the mass 'volume of the incoming air used for-scavenging, Ithe temperature of 4the prc'iduc'ts must be maintained above 133.8 when the prod# ucts iirst contact the deposited carbon dioxide, this being the equivalent of aAT of 8.2S KQ Thus,y in--thepresent examplejithe nitrogen'stream enters the cold end-of the-coldrsectionvofthe interchangerat about 85 K.and leaves the warm end Vthereof '-at about l-174` The-airstream .enters the-warm end of the-coldsection Iof -the vinterchanger atabout 1180"A and-` the icooled .carbon dioxide-freeairfstream' leaves the cold endflat about 103"E K. Thus itwill'be seen 4that at a point in the-coldsecti'on of theinterchange'r wherefthe temperature of vthe` airhas attained: Ia temperature ofsabout li142 "i K.; theca'rbonf dioxide will: beginzto 'deposit out.- Whenithe owr'of. the

iiiidslis switched, the iarwillenter fthe opposite end o'f thefcoldiseotion-inthe other passageway, thusdepositing its carbon dioxide impurity in the opposite endof the section. 'Ihe'nitrogen now enters the passageway where the air previously entered, and lthe'interchangerv is so designed that the nitrogen will be warmed to a temperature above 133.8" K. before' it reaches the zone in which the carbon dioxide has been previously deposited. At this, or any higher temperature of thenit'rogen, all of the carbon dioxide will be reinov'edwhen using. a system whereinireturning product or products in mass volume'equal to about of the incoming air is 'used for scavenging.

In the form of the invention shown in Figure 2 like iigures ldesignate similar pieces or" apparatus: In' general, the operation of the system isvsimilar to that described with respect to Figure 1 with the-exception'that the portionof the cold -compressed airA stream from thevfinterchanger owing4 to the turbo-expander is rst warmedto improve thev cycle. This is accomplished by providing an additional passageway jr through a portion or the length of the warm section 20 of the main interchanger- I9. A portion-ofthe air stream iowing through conduit 38 is ldiverted'at point 86 through conduit 81, controlled by Vvalve 88, leading to the passageway 815 through a portion of the warm section of the interchanger. As shown, passageway 8&5` passes through the lower or colder portion of Ithe warm section. From the' passageway 815iy the stream isconducted through conduit 819 to the expander S9" where the air stream is expanded while doing work and the temperature' of the gas is thus reduced.' The expanded air stream passes through conduit 'Il `to, the low pressure 'section of the column. In all other respects the operation of the systeml is identical with that described in connection with the' embodiment shown in Figure 1.

In passingA the cold' compressed air stream' through passageway 85' before' the expansion step, the stream is warmed slightly, giving up a portion of its Vcold to the warm entering air stream inthe inter'cha'nger.V By warming the cold air stream slightly prior to its passage through the turbo-expander, the latter can be operated more eiiiciently'with the liquefaction of any portion ofthe air stream being avoided to thereby increase the production of cold in the expander. The presen-ce of liquid in the turbo-expander is also objectionable in that it causes a great deal `of wear on the turbine blades requiring theirfrequent replacement.

The passage of the air stream through passageway 85 requires a slight change in the relativeeective lengths of thetwo sectionsof vthe heat interchanger, but has no elect on the method of Idepositing and purging-the carbon dioxide inthe lcold section of the interchanger. y

In thev` embodiment of the invention shown in Figure 3, the cold compressed air stream `from the interchanger does not pass through vthe lturbo-- expander.` A portion of this stream is liqueed; as previously, inthe liqueiier 65, and the balance of the-stream in gaseous form mergeswith the lqueed stream -at point 'l'l and passesinto the high pressure section of the column.

rlhe make-uprefrigeration in this embodiment is providedv by removing a stream of` high pressure nitrogen from the column through conduitr and passing this stream through the passageway 85 -in the warm section'of the'interchanger.' The stream leaves passageway throughconduit 9 I and passes to the turbo-expander 69. In the turbo-expander the pressure of the stream is re- -duced to about the pressure of the product nitrogen while doing work and the temperature of the gas isreduced. The expanded nitrogen stream then passes through conduit 92 to the point 93 where it merges with the low pressure product nitrogen stream flowing from the column through conduit 42.

In flowing through the passageway 85 the high pressure nitrogen stream is warmed so that it may be subsequently expanded without the formation of liquid in the turbo-expander.

'The operation of the carbon dioxide removal system is the same as in the embodiment shown in Figure 2.

It should be pointed out that even though a single multi-passageway heat interchanger is illustrated for both the oxygen and nitrogen products, it is possible to operate with a separate interchanger for the nitrogen product and one for the oxygen product. The airsupply could be apportioned between the two interchangers So `that the supply of air and of the returning product are equal in each interchanger, and each interchanger is operated so that the impurities are removed in accordance with the present invention. It is also possible to operate the oxygen interchanger conventionally7 with a smaller amount of incoming air than outgoing oxygen product, and applying the present invention to the nitrogen interchanger only.

dAlthough the diagrammatic views in the drawings indicate theinterchanger sections as unitary structures, obviously these sections may in practice include a plurality of heat interchanger units in series for facility in manufacture.

I claim:

17. A heat interchanger comprising a warm section having a warm end and a cold end, a cold section having a rst end anda second end, a first and second passageway through the Warm section, a first and second passageway through the coldvsecticn, a rst set of conduit means for conducting a cooling fluid to the first passageway of `the cold sectionat the first end thereof, from the first passageway of the cold section at the second end thereof tothe first passageway of the warm section at the cold end thereof, and from the rst passageway of the warm section at the warm end thereof, a second set of conduit means for conducting a cooling fluid tothe second passageway of the cold section at the second end thereof, from the second passageway of the cold section at the first end thereof to the second passageway of the warm section at the cold end thereof, and from the second passageway of the warm section at the warm 7end thereof, first switching valve means assofciated with both first and second sets of conduit `means for switching the flow of the cooling fluid 'from Vthe first set of conduit means to the secfond set of conduit means, a third set of conduit means for conducting a fluid to be cooled Yto the second passageway of the warm section at the warm end thereof 'from the second passageway of the warm sectionat the cold end thereof to the second'passageway ofthe cold section at the second end thereof and from the second passageway of the cold section at the rst end thereof, a fourth set of conduit means for conducting the'iiuid' tobe cooled to' the first passageway of the warm section at the warm end thereof .from the first passageway of the'warm section at the cold endthere'of tothe first Vpasl0 sageway' of the cold section at the rst end thereof, and from the first passageway of the 'coldf sectionat the second end thereof, second switching Vvalve means associated with both third and Vfourth sets of conduit means Vfor switching theflowtof the fluid to be cooled from the third set of conduit means to the fourth set of conduit rneans,`and means for operating the rst and second switching valve means simultaneously son that the cooling fluid and the uid to'be cooled flow through both of the first and second passageways respectively and then through both'^ofrthe second and first passageways respectively; i y Y 2; *A* heat- 4in terchanger as described in claim 1 which is utilized for cooling a mixture of gases to be fractionated andV for freezing out a higher boiling impurity-therein, the interchanger being so constructed thatthe higher boiling impurity will bedeposited inthe cold section of the interchanger. L f 3;"Apparatus' Yin accordance with claim", 2 whereinrthe cold section of the interchangeris ofsuchlengthasfto cool the `mixture of gases over v`arange at -least twice the higher boiling impurity deposition range. e 4"..In combination with air fractionating apparatus including anair fractionating column, a heat `intrchanger adapted to refrigerate and purify a 'stream of compressed air by heat interchangerwith a stream of cold expanded gaseous nitrogenfp'roduct from the fractionation ofthe air stream andby precipitation of the carbon dioxide" impurity lin the air stream, comprisingfa warm sectionhaving a warm end and a cold end, a Vcold section: havinga first end and a second end, va "first `and `second passageway throughth warm section,wa irst and second passageway through the cold section, a first set of conduit means for conducting the nitrogen product to thefirst'passageway` of the cold section atthe first end thereof,k from the first passageway'of the'coldsection at the second end thereof to the I'lrst passageway of the warm section at the cold endl thereof, land `from the rst passagewaywof the warm section at the warm end thereof,l a secondA set of conduit means for conducting the nitrogenfprod'uct to the second'passageway of the `c'oldsection at the second end thereof, Afrom the4 second passageway of the cold sectionfat the'frs't'eridthereof tothe second passageway of the Warmsection at the cold end thereof, and fromthesecond passageway of the warm section at the warm end thereof, first switching valve means associated with both sets of conduit means for -.switching the4 iiow of the nitrogen product from theiirst. s'et of conduit means to the second set :of conduit means, a third set of conduitl means'for conducting the air stream to the sec-j ond passageway of the warm section at the warm end thereof, fromi the second passageway ofthe warnrsectionfatthe cold end thereof to the second passagewayof the cold section at the second end thereof and from the second passagewayof the cold section atthe rst end thereof, a fourth set` of"conduit` means for conducting the air stream tothe `rst passageway of the warm section atthe warmgend thereof, from thev first passagewayV of the warm section at the cold'end thereof to the first,` passageway of the cold sec.- tionV at the` firstl endl thereof, and from the 4iirst` passageway of the cold section at the second end ceases of conduit means to the fourth set of conduit means, and means for Operating the first and second switching valve means simultaneously so that the nitrogen and air flow through both of the first and second passageways respectively, and then through both of the second and first passageways respectively.

5. Apparatus in accordance with claim 4 wherein the interchanger is so constructed that the carbon dioxide impurity deposits in the cold section of the interchanger.

`6. Apparatus in accordance with claim .5 wherein the cold section of the interchanger 1s of such length as to cool the air stream overa range at least twice the carbon dioxide deposition.

7. Apparatus in accordance with claim 4 comprising a third passageway through the warm section of the interchanger, and expander, and a fifth set of conduit means forcondcting a portion of the air stream leaving the cold section of the interchanger to the third passageway through the warm section of the interchanger, from the third passageway through the warm section to the' expander; and from the expander to the fraotionatingrcolumn. Y

8. Apparatus in accordance' with claim 4 comprising' a third passageway through the warm section of the interchanger', an expander, a two stage fractionating column, and a first set; of conduit means for conducting a portiorrof the gaseous nitrogen product from the higher pressure stage ofthe column tothe third passageway throughthe warm section of 'the interchanger, frein the third passageway through the warm section to the expander, and from the' expander to mergev with the nitrogen productk from the iwer pressure stage of 'the Column flowing to the coldsection of the interelianger` 9'. A heat inter'changercomprising a warm section having a warm end and a cold end and a eold section having aflrst'end and as'econd end, afirst and secondi passageway through thewarm section, a iirstv and second passageway through the c'olds'ection, warm end conduit means connecting the first" andsecond passageways of the warm section at the warm end thereof to a point of fluid-'disposal arid asource' of fluid to be cooled, respectively, switching valve means associated with the warm endcon'duit means for connecting theii'rstlpassageway with the source ofiiuid to be eooledand the' second passageway withthe point oi il' disposal, inter-'section conduitY means co ceti-iig they first fandv second passageways of th warm section at the coldend'thereof with the first andsecond passagew'ays, respectively, of thefcl'disectionf'at'the second end thereof, switch ingval-vel means associated'with thhe inter-sectorli conduit vmeans i for connecting. the first and seendp'assageways ofthe. warm section at the cold endfthereofwithfthe first' and second passagevira-ysi, respectively; oflthe' cold section at the first' ericl-the1eof;` cold end-conduit means connlfc'tirigtherstandfsecond passageways at the firstendo'fthe cold section with a source of cooling fiuidand a" p'oint'of fluid disposal, for the cooled fluid,"y respectively, switching Valve means associatedw'iththe ooldiendconduit means for connectingvtherst and second passageways at thesec'ondend .of thefcold section with a point ofiiuid disposal for the-cooled fluid and a source fc'oling fluid; respeetiveiy, whereby the` erst end `of the cold section' becomes alternately the warrnend and'. then vthe cold end thereof 10'. Apparatusl in a'ccor'dance with claim 9 which is utilized for cooling a stream of compressed air and for depositing out the carbon dioxide impurity contained therein, the warm and cold sections of the inter-changer being so proportioned that the carbon dioxide will be deposited in the cold section of the interchanger.

11. The method of continuously cooling a stream of fluid to be cooled by heat interchange with a cooling uid in a plurality of elongated heat interchange zones comprising passing the stream of fluid to be cooled through rst and second of the zones in series, passing the stream of cooling iiuid through third and fourth of the zones in series, the first and fourth zones being in heat interchange relation. with the streams therein in countercurrent flow relation and the second and thirdzonesbeing in heat interchange relation with the streams therein in countercurrentv iiow relation whereby the stream of fluid to be cooled becomes progressively cooler in passage through each Zone, switching the ow of the streams through the rst and fourth Zones so that the streamv to be cooled flows through the fourth zone and the stream of cooling fluid flows through the-first Zone but without changing the direction of flow of either of thesestreams through these zones, and simultaneously switching the flow oi the streams through the second and third'zones so that the stream of fluid tobe cooled flows through the third zone and the stream of cooling fluid flows through the second zone but with the direction of flow of both of these streams through these Zones reversedl 12. The method of claim 1-1Y in which the uid. to be cooled` is a compressed mixture of low boiling gases to be fractionated and the cooling fluid is a product off the fractionation at lower-pressure.

13. The method-of claim 12 in which the mixture of low boiling gases contains aliigher boiling impurity and in which the reduction in temperature ofY the stream of fluid to be cooled as it progresses through the zones results in the impurity being precipitated from the mixtureof low boiling point` gases in the colder portion ofthe secondY and third zones.

142 The method of claim 13 in which the mixture oflow boiling gases is cooled in the second and third zones through a temperature range at leas-t twice that in which precipitation-of the impurity occurs.

15. The Ymethod of claim 12 in which the mixture-of gases is air and the product nitrogen.

16. The method'claimed in claim 13 in which the mixture is air, the product nitrogen andthe impurity carbon dioxide.

17. The'method of claim 14 inwhich the mixture is air, the product nitrogen and the impurity carbon dioxide 18.- Inthe continuous cooling of a stream of a compressed mixture of low boiling gases containing a higher boilingnormally gaseous impurity by heat interchange with a stream of a cooling fluid ata lower pressure, in which the streams areA alternatively passed in counter currentl flow relation through heat interchange passages where the fluid to be cooled is progressively cooled first in one passage and then another to a temperature such that impurity is deposited in the one passage andthen` the other, and the cooling fluid-is passed iirst through the other passage and then through the'one passage to act as the coolant and to remove: deposited impurity, the method of insuring complete removal of the deposited `impurity by the stream of cooling fluid during flow of the cooling fluid through each passage comprising, warming up the stream of cooling fluid in such passage by heat interchange against the stream of fluid to be cooled prior to contacting the deposited impurity Withv the cooling uid so that the difference in temperature between the iiuid to be cooled when depositing the impurity and the cooling fluid when contacting the deposited impurity is less than would exist if the stream of cooling uid had not been so warmed.

19. The method of claim 18 in which the mixture of gases is air, the impurity carbon dioxide and the cooling uid a product of fractionation of air.

20. The method of continuously cooling a first stream of fluid containing a higher boiling impurity and completely removing said impurity with a second stream of cooling iuid which comprises passing the first stream of fluid through a passage in countercurrent heat exchange relation to said second stream of fluid flowing in a 14 second passage and thereby depositing said impurity toward the colder end of said rst passage and then reversing the directions of iiow of said rst and second fluid streams while interchangng their passages of flow.

21. The method of claim 20 in which the rst stream of fluid is air, the higher boiling impurity is carbon dioxide, and the second stream of fluid is a product of fractionation of air.

LEONARD PARKER POOL.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,460,859 Trumpler Feb. 8, 1949 2,490,750 Grewin Dec. 6, 1949 2,503,939 De Baufre Apr. 11, 1950 2,504,051 Scheibel Apr. 11, 1950 2,553,550 Collins May 22, 1951 2,562,812 Ogorzaly July 31, 1951 

